Web Security C. Evans
Internet-Draft C. Palmer
Intended status: Standards Track R. Sleevi
Expires: April 8, 2015 Google, Inc.
October 5, 2014
Public Key Pinning Extension for HTTPdraft-ietf-websec-key-pinning-21
Abstract
This document defines a new HTTP header that allows web host
operators to instruct user agents to remember ("pin") the hosts'
cryptographic identities over a period of time. During that time,
UAs will require that the host presents a certificate chain including
at least one Subject Public Key Info structure whose fingerprint
matches one of the pinned fingerprints for that host. By effectively
reducing the number of trusted authorities who can authenticate the
domain during the lifetime of the pin, pinning may reduce the
incidence of man-in-the-middle attacks due to compromised
Certification Authorities.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 8, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Internet-Draft Public Key Pinning Extension for HTTP October 2014Appendix B. Deployment Guidance . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 281. Introduction
This document defines a new HTTP header that enables user agents
(UAs) to determine which Subject Public Key Info (SPKI) structures
will be present in a web host's certificate chain in future TLS
[RFC5246] connections.
Deploying PKP safely will require operational and organizational
maturity due to the risk that hosts may make themselves unavailable
by pinning to a (set of) SPKI(s) that becomes invalid (see
Section 4). With care, host operators can greatly reduce the risk of
main-in-the-middle (MITM) attacks and other false-authentication
problems for their users without incurring undue risk.
PKP is meant to be used together with HTTP Strict Transport Security
(HSTS) [RFC6797], but it is possible to pin keys without requiring
HSTS.
A Pin is a relationship between a hostname and a cryptographic
identity (in this document, 1 or more of the public keys in a chain
of X.509 certificates). Pin Validation is the process a UA performs
to ensure that a host is in fact authenticated with its previously-
established Pin.
Key pinning is a trust-on-first-use (TOFU) mechanism. The first time
a UA connects to a host, it lacks the information necessary to
perform Pin Validation; UAs can only apply their normal cryptographic
identity validation. (In this document, it is assumed that UAs apply
X.509 certificate chain validation in accord with [RFC5280].)
The UA will not be able to detect and thwart a MITM attacking the
UA's first connection to the host. (However, the requirement that
the MITM provide an X.509 certificate chain that can pass the UA's
validation requirements, without error, mitigates this risk
somewhat.) Worse, such a MITM can inject its own PKP header into the
HTTP stream, and pin the UA to its own keys. To avoid post facto
detection, the attacker would have to be in a position to intercept
all future requests to the host from that UA.
Thus, key pinning as described in this document is not a perfect
defense against MITM attackers capable of passing certificate chain
validation procedures -- nothing short of pre-shared keys can be.
However, it provides significant value by allowing host operators to
limit the number of certification authorities than can vouch for the
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host's identity, and allows UAs to detect in-process MITM attacks
after the initial communication.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Server and Client Behavior2.1. Response Header Field Syntax
The "Public-Key-Pins" and "Public-Key-Pins-Report-Only" header
fields, also referred to within this specification as the PKP and
PKP-RO header fields, respectively, are new response headers defined
in this specification. They are used by a server to indicate that a
UA should perform Pin Validation (Section 2.6) for the host emitting
the response message, and to provide the necessary information for
the UA to do so.
Figure 1 describes the syntax (Augmented Backus-Naur Form) of the
header fields, using the grammar defined in [RFC5234] and the rules
defined in Section 3.2 of [RFC7230]. The field values of both header
fields conform to the same rules.
Public-Key-Directives = directive *( OWS ";" OWS directive )
directive = directive-name [ "=" directive-value ]
directive-name = token
directive-value = token
/ quoted-string
Figure 1: HPKP Header Syntax
Optional white space (OWS) is used as defined in Section 3.2.3 of
[RFC7230]. token and quoted-string are used as defined in
Section 3.2.6 of [RFC7230].
The directives defined in this specification are described below.
The overall requirements for directives are:
1. The order of appearance of directives is not significant.
2. A given directive MUST NOT appear more than once in a given
header field. Directives are either optional or required, as
stipulated in their definitions.
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3. Directive names are case-insensitive.
4. UAs MUST ignore any header fields containing directives, or other
header field value data, that do not conform to the syntax
defined in this specification. In particular, UAs must not
attempt to fix malformed header fields.
5. If a header field contains any directive(s) the UA does not
recognize, the UA MUST ignore those directives.
6. If the PKP or PKP-RO header field otherwise satisfies the above
requirements (1 through 5), the UA MUST process the directives it
recognizes.
Additional directives extending the semantic functionality of the
header fields can be defined in other specifications. The first such
specification will need to define a registry for such directives.
Such future directives will be ignored by UAs implementing only this
specification, as well as by generally non-conforming UAs.
When a connection passes Pin Validation using the UA's noted Pins for
the host at the time, the host becomes a Known Pinned Host.
2.1.1. The pin- Directive
The pin directive specifies a way for web host operators to indicate
a cryptographic identity that should be bound to a given web host.
The syntax of a pin directive is as follows:
pin-directive = pin-directive-name "=" pin-directive-value
pin-directive-name = "pin-" token
pin-directive-value = quoted-string
Figure 2: Pin Directive Syntax
In the pin-directive, the token is the name of a cryptographic hash
algorithm. The only algorithm allowed at this time is "sha256", i.e.
the hash algorithm SHA256 ([RFC4634]); additional algorithms may be
allowed for use in this context in the future. The quoted-string is
a sequence of base 64 digits: the base 64-encoded SPKI Fingerprint
[RFC4648] (see Section 2.4).
According to the processing rules of Section 2.1, the UA MUST ignore
pin-directives with tokens naming hash algorithms it does not
recognize. If the set of remaining effective pin-directives is
empty, and if the host is a Known Pinned Host, the UA MUST cease to
consider the host as a Known Pinned Host (the UA should fail open).
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The UA should indicate to users that the host is no longer a Known
Pinned Host.
Note, per the processing rules of Section 2.1, the pin-directive-name
is case insensitive.
2.1.2. The max-age Directive
The "max-age" directive specifies the number of seconds after the
reception of the PKP header field during which the UA SHOULD regard
the host (from whom the message was received) as a Known Pinned Host.
The "max-age" directive is REQUIRED to be present within a "Public-
Key-Pins" header field. The "max-age" directive is meaningless
within a "Public-Key-Pins-Report-Only" header field, and UAs MUST
ignore it and not cache the header. See Section 2.3.3.
The max-age directive is REQUIRED to have a directive value, for
which the syntax (after quoted-string unescaping, if necessary) is
defined as:
max-age-value = delta-seconds
delta-seconds = 1*DIGIT
Figure 3: max-age Value Syntax
delta-seconds is used as defined in [RFC7234], Section 1.2.1.
See Section 2.3.3 for limitations on the range of values for max-age.
2.1.3. The includeSubDomains Directive
The OPTIONAL includeSubDomains directive is a valueless directive
that, if present (i.e., it is "asserted"), signals to the UA that the
Pinning Policy applies to this Pinned Host as well as any subdomains
of the host's domain name.
2.1.4. The report-uri Directive
The OPTIONAL report-uri directive indicates the URI to which the UA
SHOULD report Pin Validation failures (Section 2.6). The UA POSTs
the reports to the given URI as described in Section 3.
When used in the PKP or PKP-RO headers, the presence of a report-uri
directive indicates to the UA that in the event of Pin Validation
failure it SHOULD POST a report to the report-uri. If the header is
Public-Key-Pins, the UA should do this in addition to terminating the
connection (as described in Section 2.6).
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Hosts may set report-uris that use HTTP or HTTPS. If the scheme in
the report-uri is one that uses TLS (e.g. HTTPS), UAs MUST perform
Pinning Validation when the host in the report-uri is a Known Pinned
Host; similarly, UAs MUST apply HSTS if the host in the report-uri is
a Known HSTS Host.
Note that the report-uri need not necessarily be in the same Internet
domain or web origin as the host being reported about.
UAs SHOULD make their best effort to report Pin Validation failures
to the report-uri, but may fail to report in exceptional conditions.
For example, if connecting the report-uri itself incurs a Pinning
Validation failure or other certificate validation failure, the UA
MUST cancel the connection. Similarly, if Known Pinned Host A sets a
report-uri referring to Known Pinned Host B, and if B sets a report-
uri referring to A, and if both hosts fail Pin Validation, the UA
SHOULD detect and break the loop by failing to send reports to and
about those hosts.
In any case of report failure, the UA MAY attempt to re-send the
report later.
UAs SHOULD limit the rate at which they send reports. For example,
it is unnecessary to send the same report to the same report-uri more
than once per distinct set of declared Pins.
2.1.5. Examples
Figure 4 shows some example PKP and PKP-RO response header fields.
(Lines are folded to fit.)
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Establishing a given host as a Known Pinned Host, in the context of a
given UA, is accomplished as follows:
1. Over the HTTP protocol running over secure transport, by
correctly returning (per this specification) at least one valid
PKP header field to the UA.
2. Through other mechanisms, such as a client-side pre-loaded Known
Pinned Host List.
2.2.2. HTTP Request Type
Pinned Hosts SHOULD NOT include the PKP header field in HTTP
responses conveyed over non-secure transport. UAs MUST ignore any
PKP header received in an HTTP response conveyed over non-secure
transport.
2.3. User Agent Processing Model
The UA processing model relies on parsing domain names. Note that
internationalized domain names SHALL be canonicalized according to
the scheme in Section 10 of [RFC6797].
2.3.1. Public-Key-Pins Response Header Field Processing
If the UA receives, over a secure transport, an HTTP response that
includes a PKP header field conforming to the grammar specified in
Section 2.1, and there are no underlying secure transport errors or
warnings (see Section 2.5), the UA MUST either:
o Note the host as a Known Pinned Host if it is not already so noted
(see Section 2.3.3),
or,
o Update the UA's cached information for the Known Pinned Host if
any of of the max-age, includeSubDomains, or report-uri header
field value directives convey information different from that
already maintained by the UA.
The max-age value is essentially a "time to live" value relative to
the time of the most recent observation of the PKP header field. If
the max-age header field value token has a value of 0, the UA MUST
remove its cached Pinning Policy information (including the
includeSubDomains directive, if asserted) if the Pinned Host is
Known, or, MUST NOT note this Pinned Host if it is not yet Known.
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If a UA receives more than one PKP header field or more than one PKP-
RO header field in an HTTP response message over secure transport,
then the UA MUST process only the first PKP header field (if present)
and only the first PKP-RO header field (if present).
If the UA receives the HTTP response over insecure transport, or if
the PKP header is not a Valid Pinning Header (see Section 2.5), the
UA MUST ignore any present PKP header field(s). Similarly, if the UA
receives the HTTP response over insecure transport, the UA MUST
ignore any present PKP-RO header field(s). The UA MUST ignore any
PKP or PKP-RO header fields not conforming to the grammar specified
in Section 2.1.
2.3.2. Interaction of Public-Key-Pins and Public-Key-Pins-Report-Only
A server MAY set both the "Public-Key-Pins" and "Public-Key-Pins-
Report-Only" headers simultaneously. The headers do not interact
with one another but the UA MUST process the PKP header and SHOULD
process both.
The headers are processed according to Section 2.3.1.
When the PKP-RO header is used with a report-uri, the UA SHOULD POST
reports for Pin Validation failures to the indicated report-uri,
although the UA MUST NOT enforce Pin Validation. That is, in the
event of Pin Validation failure when the host has set the PKP-RO
header, the UA performs Pin Validation only to check whether or not
it should POST a report, but not for causing connection failure.
Note: There is no purpose to using the PKP-RO header without the
report-uri directive. User Agents MAY discard such headers without
interpreting them further.
When the PKP header is used with a report-uri, the UA SHOULD POST
reports for Pin Validation failures to the indicated report-uri, as
well as enforcing Pin Validation.
If a host sets the PKP-RO header, the UA SHOULD note the Pins and
directives given in the PKP-RO header, ignoring any max-age
directive. If the UA does note the Pins and directives in the PKP-RO
header it SHOULD evaluate the specified policy and SHOULD report any
would-be Pin Validation failures that would occur if the report-only
policy were enforced.
If a host sets both the PKP header and the PKP-RO header, the UA MUST
note and enforce Pin Validation as specified by the PKP header, and
SHOULD process the Pins and directives given in the PKP-RO header.
If the UA does process the Pins and directives in the PKP-RO header
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it SHOULD evaluate the specified policy and SHOULD report any would-
be Pin Validation failures that would occur if the report-only policy
were enforced.
2.3.3. Noting a Pinned Host - Storage Model
The Effective Pin Date of a Known Pinned Host is the time that the UA
observed a Valid Pinning Header for the host. The Effective
Expiration Date of a Known Pinned Host is the Effective Pin Date plus
the max-age. A Known Pinned Host is "expired" if the Effective
Expiration Date refers to a date in the past. The UA MUST ignore any
expired Known Pinned Hosts in its cache.
For example, if a UA is beginning to perform Pin Validation for a
Known Pinned Host and finds that the cached pinning information for
the host indicates an Effective Expiration Date in the past, the UA
MUST NOT continue with Pin Validation for the host, and must consider
the host to no longer be a Known Pinned Host.
Known Pinned Hosts are identified only by domain names, and never IP
addresses. If the substring matching the host production from the
Request-URI (of the message to which the host responded)
syntactically matches the IP-literal or IPv4address productions from
Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a
Known Pinned Host.
Otherwise, if the substring does not congruently match an existing
Known Pinned Host's domain name, per the matching procedure specified
in Section 8.2 of [RFC6797], then the UA MUST add this host to the
Known Pinned Host cache. The UA caches:
o the Pinned Host's domain name,
o the Effective Expiration Date, or enough information to calculate
it (the Effective Pin Date and the value of the max-age
directive),
o whether or not the includeSubDomains directive is asserted, and
o the value of the report-uri directive, if present.
If any other metadata from optional or future PKP header directives
are present in the Valid Pinning Header, and the UA understands them,
the UA MAY note them as well.
UAs MAY set an upper limit on the value of max-age, so that UAs that
have noted erroneous Pins (whether by accident or due to attack) have
some chance of recovering over time. If the server sets a max-age
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greater than the UA's upper limit, the UA MAY behave as if the server
set the max-age to the UA's upper limit. For example, if the UA caps
max-age at 5184000 seconds (60 days), and a Pinned Host sets a max-
age directive of 90 days in its Valid Pinning Header, the UA MAY
behave as if the max-age were effectively 60 days. (One way to
achieve this behavior is for the UA to simply store a value of 60
days instead of the 90 day value provided by the Pinned Host.) For
UA implementation guidance on how to select a maximum max-age, see
Section 4.1.
The UA MUST NOT modify any pinning metadata of any superdomain
matched Known Pinned Host.
The UA MUST NOT cache information derived from a PKP-RO header.
(PKP-RO headers are useful only at the time of receipt and
processing.)
2.3.4. HTTP-Equiv <Meta> Element Attribute
UAs MUST NOT heed http-equiv="Public-Key-Pins" or http-equiv="Public-
Key-Pins-Report-Only" attribute settings on <meta> elements
[W3C.REC-html401-19991224] in received content.
2.4. Semantics of Pins
An SPKI Fingerprint is defined as the output of a known cryptographic
hash algorithm whose input is the DER-encoded ASN.1 representation of
the subjectPublicKeyInfo (SPKI) field of an X.509 certificate. A Pin
is defined as the combination of the known algorithm identifier and
the SPKI Fingerprint computed using that algorithm.
The SPKI Fingerprint is encoded in base 64 for use in an HTTP header
[RFC4648].
In this version of the specification, the known cryptographic hash
algorithm is SHA-256, identified as "sha256" [RFC6234]. (Future
specifications may add new algorithms and deprecate old ones.) UAs
MUST ignore Pins for which they do not recognize the algorithm
identifier. UAs MUST continue to process the rest of a PKP response
header field and note Pins for algorithms they do recognize.
Figure 5 reproduces the definition of the SubjectPublicKeyInfo
structure in [RFC5280].
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SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
Figure 5: SPKI Definition
If the certificate's subjectPublicKeyInfo is incomplete when taken in
isolation, such as when holding a DSA key without domain parameters,
a public key pin cannot be formed.
We pin public keys, rather than entire certificates, to enable
operators to generate new certificates containing old public keys
(see [why-pin-key]).
See Appendix A for an example non-normative program that generates
SPKI Fingerprints from certificates.
2.5. Noting Pins
Upon receipt of the PKP response header field, the UA notes the host
as a Known Pinned Host, storing the Pins and their associated
directives in non-volatile storage (for example, along with the HSTS
metadata). The Pins and their associated directives are collectively
known as Pinning Metadata.
The UA MUST note the Pins for a Host if and only if all three of the
following conditions hold:
o It received the PKP response header field over an error-free TLS
connection. If the host is a Pinned Host, this includes the
validation added in Section 2.6.
o The TLS connection was authenticated with a certificate chain
containing at least one of the SPKI structures indicated by at
least one of the given SPKI Fingerprints (see Section 2.6).
o The given set of Pins contains at least one Pin that does NOT
refer to an SPKI in the certificate chain. (That is, the host
must set a Backup Pin; see Section 4.3.)
If the PKP response header field does not meet all three of these
criteria, the UA MUST NOT note the host as a Pinned Host. A PKP
response header field that meets all these critera is known as a
Valid Pinning Header.
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Whenever a UA receives a Valid Pinning Header, it MUST set its
Pinning Metadata to the exact Pins, Effective Expiration Date
(computed from max-age), and (if any) report-uri given in the most
recently received Valid Pinning Header.
For forward compatibility, the UA MUST ignore any unrecognized PKP
and PKP-RO header directives, while still processing those directives
it does recognize. Section 2.1 specifies the directives max-age,
Pins, includeSubDomains, and report-uri but future specifications and
implementations might use additional directives.
Upon receipt of a PKP-RO response header field, the UA SHOULD
evaluate the policy expressed in the field, and SHOULD generate and
send a report (see Section 3). However, failure to validate the Pins
in the field MUST have no effect on the validity or non-validity of
the policy expressed in the PKP field or in previously-noted Pins for
the Known Pinned Host.
The UA need not note any Pins or other policy expressed in the PKP-RO
response header field, except for the purpose of determining that it
has already sent a report for a given policy. UAs SHOULD make a best
effort not to inundate report-uris with redundant reports.
2.6. Validating Pinned Connections
When a UA connects to a Pinned Host using a TLS connection, if the
TLS connection has errors, the UA MUST terminate the connection
without allowing the user to proceed anyway. (This behavior is the
same as that required by [RFC6797].)
If the connection has no errors, then the UA will determine whether
to apply a new, additional correctness check: Pin Validation. A UA
SHOULD perform Pin Validation whenever connecting to a Known Pinned
Host, as soon as possible (e.g. immediately after receiving the
Server Certificate message). It is acceptable to allow Pin
Validation to be disabled for some Hosts according to local policy.
For example, a UA may disable Pin Validation for Pinned Hosts whose
validated certificate chain terminates at a user-defined trust
anchor, rather than a trust anchor built-in to the UA (or underlying
platform).
To perform Pin Validation, the UA will compute the SPKI Fingerprints
for each certificate in the Pinned Host's validated certificate
chain, using each supported hash algorithm for each certificate. (As
described in Section 2.4, certificates whose SPKI cannot be taken in
isolation cannot be pinned.) The UA MUST ignore superfluous
certificates in the chain that do not form part of the validating
chain. The UA will then check that the set of these SPKI
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Fingerprints intersects the set of SPKI Fingerprints in that Pinned
Host's Pinning Metadata. If there is set intersection, the UA
continues with the connection as normal. Otherwise, the UA MUST
treat this Pin Validation Failure as a non-recoverable error. Any
procedure that matches the results of this Pin Validation procedure
is considered equivalent.
A UA that has previously noted a host as a Known Pinned Host MUST
perform Pin Validation when setting up the TLS session, before
beginning an HTTP conversation over the TLS channel.
UAs send validation failure reports only when Pin Validation is
actually in effect. Pin Validation might not be in effect e.g.
because the user has elected to disable it, or because a presented
certificate chain chains up to a user-defined trust anchor. In such
cases, UAs SHOULD NOT send reports.
2.7. Interactions With Preloaded Pin Lists
UAs MAY choose to implement additional sources of pinning
information, such as through built-in lists of pinning information.
Such UAs should allow users to override such additional sources,
including disabling them from consideration.
The effective policy for a Known Pinned Host that has both built-in
Pins and Pins from previously observed PKP header response fields is
implementation-defined.
2.8. Pinning Self-Signed End Entities
If UAs accept hosts that authenticate themselves with self-signed end
entity certificates, they MAY also allow hosts to pin the public keys
in such certificates. The usability and security implications of
this practice are outside the scope of this specification.
3. Reporting Pin Validation Failure
When a Known Pinned Host has set the report-uri directive, the UA
SHOULD report Pin Validation failures to the indicated URI. The UA
does this by POSTing a JSON [RFC7159] message to the URI; the JSON
message takes this form:
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{
"date-time": date-time,
"hostname": hostname,
"port": port,
"effective-expiration-date": expiration-date,
"include-subdomains": include-subdomains,
"noted-hostname": noted-hostname,
"served-certificate-chain": [
pem1, ... pemN
],
"validated-certificate-chain": [
pem1, ... pemN
],
"known-pins": [
known-pin1, ... known-pinN
]
}
Figure 6: JSON Report Format
Whitespace outside of quoted strings is not significant. The key/
value pairs may appear in any order, but each MUST appear only once.
The date-time indicates the time the UA observed the Pin Validation
failure. It is provided as a string formatted according to
Section 5.6, "Internet Date/Time Format", of [RFC3339].
The hostname is the hostname to which the UA made the original
request that failed Pin Validation. It is provided as a string.
The port is the port to which the UA made the original request that
failed Pin Validation. It is provided as an integer.
The effective-expiration-date is the Effective Expiration Date for
the noted Pins. It is provided as a string formatted according to
Section 5.6, "Internet Date/Time Format", of [RFC3339].
include-subdomains indicates whether or not the UA has noted the
includeSubDomains directive for the Known Pinned Host. It is
provided as one of the JSON identifiers "true" or "false".
noted-hostname indicates the hostname that the UA noted when it noted
the Known Pinned Host. This field allows operators to understand why
Pin Validation was performed for e.g. foo.example.com when the noted
Known Pinned Host was example.com with includeSubDomains set.
The served-certificate-chain is the certificate chain, as served by
the Known Pinned Host during TLS session setup. It is provided as an
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array of strings; each string pem1, ... pemN is the PEM
representation of each X.509 certificate as described in
[I-D.josefsson-pkix-textual].
The validated-certificate-chain is the certificate chain, as
constructed by the UA during certificate chain verification. (This
may differ from the served-certificate-chain.) It is provided as an
array of strings; each string pem1, ... pemN is the PEM
representation of each X.509 certificate as described in
[I-D.josefsson-pkix-textual]. For UAs that build certificate chains
in more than one way during the validation process, they SHOULD send
the last chain built. In this way they can avoid keeping too much
state during the validation process.
The known-pins are the Pins that the UA has noted for the Known
Pinned Host. They are provided as an array of strings with the
syntax:
known-pin = token "=" quoted-string
Figure 7: Known Pin Syntax
As in Section 2.4, the token refers to the algorithm name, and the
quoted-string refers to the base 64 encoding of the SPKI Fingerprint.
When formulating the JSON POST body, the UA MUST either use single-
quoted JSON strings, or use double-quoted JSON strings and \-escape
the embedded double quotes in the quoted-string part of the known-
pin.
Figure 8 shows an example of a Pin Validation failure report. (PEM
strings are shown on multiple lines for readability.)
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{
"date-time": "2014-04-06T13:00:50Z",
"hostname": "www.example.com",
"port": 443,
"effective-expiration-date": "2014-05-01T12:40:50Z"
"include-subdomains": false,
"served-certificate-chain": [
"-----BEGIN CERTIFICATE-----\n
MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
...
HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
-----END CERTIFICATE-----",
...
],
"validated-certificate-chain": [
"-----BEGIN CERTIFICATE-----\n
MIIEBDCCAuygAwIBAgIDAjppMA0GCSqGSIb3DQEBBQUAMEIxCzAJBgNVBAYTAlVT\n
...
HFa9llF7b1cq26KqltyMdMKVvvBulRP/F/A8rLIQjcxz++iPAsbw+zOzlTvjwsto\n
WHPbqCRiOwY1nQ2pM714A5AuTHhdUDqB1O6gyHA43LL5Z/qHQF1hwFGPa4NrzQU6\n
yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx\n
-----END CERTIFICATE-----",
...
],
"known-pins": [
'pin-sha256="d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="',
"pin-sha256=\"E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=\""
]
}
Figure 8: Pin Validation Failure Report Example
4. Security Considerations
Pinning public keys helps hosts strongly assert their cryptographic
identity even in the face of issuer error, malfeasance or compromise.
But there is some risk that a host operator could lose or lose
control of their host's private key (such as by operator error or
host compromise). If the operator had pinned only the key of the
host's end entity certificate, the operator would not be able to
serve their web site or application in a way that UAs would trust for
the duration of their pin's max-age. (Recall that UAs MUST close the
connection to a host upon Pin Failure.)
Therefore, there is a necessary trade-off between two competing
goods: pin specificity and maximal reduction of the scope of issuers
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on the one hand; and flexibility and resilience of the host's
cryptographic identity on the other hand. One way to resolve this
trade-off is to compromise by pinning to the key(s) of the issuer(s)
of the host's end entity certificate(s). Often, a valid certificate
chain will have at least two certificates above the end entity
certificate: the intermediate issuer, and the trust anchor.
Operators can pin any one or more of the public keys in this chain,
and indeed MUST pin to issuers not in the chain (as, for example, a
Backup Pin). Pinning to an intermediate issuer, or even to a trust
anchor or root, still significantly reduces the number of issuers who
can issue end entity certificates for the Known Pinned Host, while
still giving that host flexibility to change keys without a
disruption of service.
4.1. Maximum max-age
As mentioned in Section 2.3.3, UAs MAY cap the max-age value at some
upper limit. There is a security trade-off in that low maximum
values provide a narrow window of protection for users who visit the
Known Pinned Host only infrequently, while high maximum values might
potentially result in a UA's inability to successfully perform Pin
Validation for a Known Pinned Host if the UA's noted Pins and the
host's true Pins diverge.
Such divergence could occur for several reasons, including: UA error;
host operator error; network attack; or a Known Pinned Host that
intentionally migrates all pinned keys, combined with a UA that has
noted true Pins with a high max-age value and has not had a chance to
observe the new true Pins for the host. (This last example
underscores the importance for host operators to phase in new keys
gradually, and to set the max-age value in accordance with their
planned key migration schedule.)
There is probably no ideal upper limit to the max-age directive that
would satisfy all use cases. However, a value on the order of 60
days (5,184,000 seconds) may be considered a balance between the two
competing security concerns.
4.2. Using includeSubDomains Safely
It may happen that Pinned Hosts whose hostnames share a parent domain
use different Valid Pinning Headers. If a host whose hostname is a
parent domain for another host sets the includeSubDomains directive,
the two hosts' Pins may conflict with each other. For example,
consider two Known Pinned Hosts, example.com and
subdomain.example.com. Assume example.com sets a Valid Pinning
Header such as this:
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Public-Key-Pins: max-age=12000; pin-sha256="ABC...";
pin-sha256="DEF..."; includeSubDomains
Figure 9: example.com Valid Pinning Header
Assume subdomain.example.com sets a Valid Pinning Header such as
this:
Public-Key-Pins: pin-sha256="GHI..."; pin-sha256="JKL..."
Figure 10: subdomain.example.com Valid Pinning Header
Assume a UA that has not previously noted any Pins for either of
these hosts. If the UA first contacts subdomain.example.com, it will
note the Pins in the Valid Pinning Header, and perform Pin Validation
as normal on subsequent conections. If the UA then contacts
example.com, again it will note the Pins and perform Pin Validation
on future connections.
However, if the UA happened to visit example.com before
subdomain.example.com, the UA would, due to example.com's use of the
includeSubDomains directive, attempt to perform Pin Validation for
subdomain.example.com using the SPKI hashes ABC... and DEF..., which
are not valid for the certificate chains subdomain.example.com (which
uses certificates with SPKIs GHI... and JLK...). Thus, depending on
the order in which the UA observes the Valid Pinning Headers for
hosts example.com and subdomain.example.com, Pin Validation might or
might not fail for subdomain.example.com, even if the certificate
chain the UA receives for subdomain.example.com is perfectly valid.
Thus, Pinned Host operators must use the includeSubDomains directive
with care. For example, they may choose to use overlapping pin sets
for hosts under a parent domain that uses includeSubDomains, or to
not use the includeSubDomains directive in their effective-second-
level domains, or to simply use the same pin set for all hosts under
a given parent domain.
4.3. Backup Pins
The primary way to cope with the risk of inadvertent Pin Validation
Failure is to keep a Backup Pin. A Backup Pin is a fingerprint for
the public key of a secondary, not-yet-deployed key pair. The
operator keeps the backup key pair offline, and sets a pin for it in
the PKP header. Then, in case the operator loses control of their
primary private key, they can deploy the backup key pair. UAs, who
have had the backup key pair pinned (when it was set in previous
Valid Pinning Headers), can connect to the host without error.
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Because having a backup key pair is so important to recovery, UAs
MUST require that hosts set a Backup Pin (see Section 2.5). The down
side of keeping a not-yet-deployed key pair is that if an attacker
gains control of the private key she will be able to perform a MITM
attack without being discovered. Operators must take care to avoid
leaking the key such as keeping it offline.
4.4. Interactions With Cookie Scoping
HTTP cookies [RFC6265] set by a Known Pinned Host can be stolen by a
network attacker who can forge web and DNS responses so as to cause a
client to send the cookies to a phony subdomain of the host. To
prevent this, hosts SHOULD set the "secure" attribute and precisely
scope the "domain" attribute on all security-sensitive cookies, such
as session cookies. These settings tell the browser that the cookie
should only be sent back to the specific host(s) (and not e.g. all
subdomains of a given domain), and should only be sent over HTTPS
(not HTTP).
4.5. Hostile Pinning
An attacker who is able to obtain a valid certificate for a domain,
either through misissuance by a Certification Authority or through
other means, such as being the prior owner of a given domain, may
attempt to perform 'hostile' pinning. In this scenario, the attacker
provides a Valid Pinning Header that pins to a set of SPKIs of the
attacker's choice. If a UA has not previously noted pins for that
host, it may note the attacker's pins, preventing access to the
legitimate site.
This attack is mitigated through several means. Most prominantly,
the attack can only persist for the maximum max-age (see
Section 4.1). Web host operators can reduce the opportunity for
attack by working to preload the host's pins within the UA.
Operators may further detect such misissuance through other means,
such as Certificate Transparency ([RFC6962]).
5. Privacy Considerations
Hosts can use HSTS or HPKP as a "super-cookie", by setting distinct
policies for a number of subdomains. For example, assume example.com
wishes to track distinct UAs without explicitly setting a cookie, or
if a previously-set cookie is deleted from the UA's cookie store.
Here are two attack scenarios.
o example.com can use report-uri and the ability to pin arbitrary
identifiers to distinguish UAs.
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1. example.com sets a Valid Pinning Header in its response to
requests. The header asserts the includeSubDomains directive,
and specifies a report-uri directive as well. Pages served by
the host also include references to subresource
https://bad.example.com/foo.png.
2. The Valid Pinning Header includes a "pin" that is not really
the hash of an SPKI, but is instead an arbitrary
distinguishing string sent only in response to a particular
request. For each request, the host creates a new, distinct
distinguishing string and sets it as if it were a pin.
3. The certificate chain served by bad.example.com does not pass
Pin Validation given the pin set the host asserted in (1).
The HPKP-conforming UA attempts to report the Pin Validation
failure to the specified report-uri, including the certificate
chain it observed and the SPKI hashes it expected to see.
Among the SPKI hashes is the distinguishing string in step
(2).
o Different site operators/origins can optionally collaborate by
setting the report-uri to be in an origin they share
administrative control of. UAs MAY, therefore, refuse to send
reports outside of the origin that set the PKP or PKP-RO header.
o example.com can use server name indication (SNI; [RFC3546]) and
subdomains to distinguish UAs.
1. example.com sets a Valid Pinning Header in its response to
requests. The header asserts the includeSubDomains directive.
2. On a subsequent page view, the host responds with a page
including the subresource https://0.fingerprint.example.com/foo.png, and the server responds using a certificate chain
that does not pass Pin Validation for the pin-set defined in
the Valid Pinning Header in step (1). The HPKP-conforming UA
will close the connection, never completing the request to
0.fingerprint.example.com. The host may thus note that this
particular UA had noted the (good) Pins for that subdomain.
3. example.com can distinguish 2^N UAs by serving Valid Pinning
Headers from an arbitrary number N distinct subdomains. For
any given subdomain n.fingerprint.example.com, the host may
deliver a Valid Pinning Header to one UA, but not deliver it
to a different UA. The server may then change the
configuration for n.fingerprint.example.com. If the UA fails
to connect, it was in the set of UAs that were pinned, which
can be distinguished from the UAs that were not pinned, as
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they will succeed in connecting. The host may repeat this for
a sufficient number of subdomains necessary to distinguish
individual UAs.
o Conforming implementations (as well as implementations conforming
to [RFC6797]) must store state about which domains have set
policies, hence which domains the UA has contacted. Because these
policies cause remotely-detectable behaviours, it is advisable
that UAs have a way for privacy-sensitive users to clear current
Pins for Pinned Hosts, and to allow users to query the current
state of Pinned Hosts. In addition, note that because Pinning a
Host implies a degree of persistent state, an attacker with
physical access to a device may be able to recover information
about hosts a user has visited, even if the user has cleared other
parts of the UA's state.
o Pin reports, as noted in Section 3, contains information about the
certificate chain that has failed pin validation. In some cases,
such as organization-wide compromise of the end-to-end security of
TLS, this may include information about the interception tools and
design used by the organization that the organization would
otherwise prefer not be disclosed.
6. IANA Considerations
IANA is requested to register the response headers described in this
document in the "Message Headers" registry ([permanent-headers] with
the following parameters:
o Header Field Names should be "Public-Key-Pins" and "Public-Key-
Pins-Report-Only".
o Protocol should be "http"
o Status should be "standard"
o Reference should be this document
7. Usability Considerations
When pinning works to detect impostor Pinned Hosts, users will
experience denial of service. It is advisable for UAs to explain the
reason why, i.e. that it was impossible to verify the confirmed
cryptographic identity of the host.
It is advisable that UAs have a way for users to clear current Pins
for Pinned Hosts, and to allow users to query the current state of
Pinned Hosts.
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Internet-Draft Public Key Pinning Extension for HTTP October 20148. Acknowledgements
Thanks to Tobias Gondrom, Jeff Hodges, Paul Hoffman, Ivan Krstic,
Adam Langley, Barry Leiba, Nicolas Lidzborski, SM, James Manger, Yoav
Nir, Trevor Perrin, Eric Rescorla, Pete Resnick, Tom Ritter, and Yan
Zhu for suggestions and edits that clarified the text.
9. What's Changed
[RFC EDITOR: PLEASE REMOVE THIS SECTION]
Added attack scenario for hostile pinning, as well as mitigations.
Added privacy considerations for the report-uri processing.
Moved pin-directive into its own directive section, leaving the
header syntax to only define directive-name and directive-value.
Updated Privacy Considerations to note that UAs should offer ways to
clear data, and in doing so, clarified the term 'forensic attacker'
to indicate an attacker with physical access.
Updated ABNF for Public-Key-Directives to indicate that at least one
directive is required, and that a directive is required between each
semi-colon.
Clarified that max-age is REQUIRED for PKP, but OPTIONAL for PKP-RO
(where it has no effect.
Updated header field syntax and description to match that in
[RFC7230].
Updated normative references to current documents.
Removed the strict directive.
Removed the requirement that the server set the Valid Pinning Header
on every response.
Added normative references for SHA, JSON, and base-64.
Added the Privacy Considerations section.
Changed non-normative pin generation code from Go to POSIX shell
script using openssl.
Changed max-max-age from SHOULD to MAY, and used the example of 60
days instead of 30.
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[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, June 2013.
[why-pin-key]
Langley, A., "Public Key Pinning", May 2011,
<http://www.imperialviolet.org/2011/05/04/pinning.html>.
Appendix A. Fingerprint Generation
This POSIX shell program generates SPKI Fingerprints, suitable for
use in pinning, from PEM-encoded certificates. It is non-normative.
openssl x509 -noout -in certificate.pem -pubkey | \
openssl asn1parse -noout -inform pem -out public.key
openssl dgst -sha256 -binary public.key | openssl enc -base64
Figure 11: Example SPKI Fingerprint Generation Code
Appendix B. Deployment Guidance
This section is non-normative guidance which may smooth the adoption
of public key pinning.
o Operators should get the backup public key signed by a different
(root and/or intermediary) CA than their primary certificate, and
store the backup key pair safely offline. The semantics of an
SPKI Fingerprint do not require the issuance of a certificate to
construct a valid Pin. However, in many deployment scenarios, in
order to make a Backup Pin operational the server operator will
need to have a certificate to deploy TLS on the host. Failure to
obtain a certificate through prior arrangement will leave clients
that recognize the site as a Known Pinned Host unable to
successfully perform Pin Validation until such a time as the
operator can obtain a new certificate from their desired
certificate issuer.
o It is most economical to have the backup certificate signed by a
completely different signature chain than the live certificate, to
maximize recoverability in the event of either root or
intermediary signer compromise.
o Operators should periodically exercise their Backup Pin plan -- an
untested backup is no backup at all.
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